1. Field of the Invention
The present invention relates generally to a Group-III-element nitride crystal semiconductor device.
2. Related Background Art
A Group-III-element nitride compound semiconductor such as, for instance, gallium nitride (GaN) (hereinafter also referred to as a “Group-III-element nitride semiconductor” or a “GaN-based semiconductor”) has been gaining attention as a material for semiconductor elements that emit green, blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disk devices or displays while a light emitting diode (LED) that emits blue light is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of, for instance, biotechnology and an ultraviolet LED as, for example, an ultraviolet source for a fluorescent lamp.
Usually, substrates made of a Group-III-element nitride semiconductor (for instance, GaN) that are used for LDs or LEDs are formed by heteroepitaxially growing Group-III-element nitride crystals on a sapphire substrate using vapor phase epitaxy. Generally, crystals obtained by this method have a dislocation density of 108 cm−2 to 109 cm−2. The reduction in dislocation density therefore has been an important issue. In order to resolve this issue, various studies have been made to reduce the dislocation density and for instance, an epitaxial lateral overgrowth (ELOG) method has been developed. With this method, the dislocation density can be reduced to about 105 cm−2 to 106 cm−2. This method, however, includes complicated processes.
On the other hand, besides the vapor phase epitaxy, a method of growing crystals from a liquid phase also has been studied. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm (10000×1.013×105 Pa) at the melting point of Group-III-element nitride single crystals such as GaN or AlN, conditions of 8000 atm (8000×1.013×105 Pa) at 1200° C. conventionally are required for growing GaN crystals from a liquid phase. Recently, however, it has been confirmed that the use of a Na flux allows GaN crystals to be synthesized at relatively low temperature and pressure, specifically, 750° C. and 50 atm (50×1.013×105 Pa).
Recently, single crystals whose maximum crystal size is about 1.2 mm are obtained by a method in which a mixture of Ga and Na is melted in a nitrogen gas atmosphere containing ammonia at 800° C. and 50 atm (50×1.013×105 Pa), and then crystals are grown for 96 hours using the melt (see, for instance, JP2002-293696A).
Furthermore, another method has been reported in which a GaN crystal layer is formed on a sapphire substrate by a metalorganic chemical vapor deposition (MOCVD) method and then single crystals are grown by a liquid phase epitaxy (LPE) method.
In the LPE method by which nitride crystals are grown using at least one of alkali metal and alkaline-earth metal as a flux, however, the introduction of impurities is a problem since it may change, for instance, the carrier density. In addition, the alkali metal and alkaline-earth metal to be used as a flux are elements that are particularly susceptible to the introduction of impurities in the semiconductor processes.